1. Field of the Invention
The present invention relates to an antiskid apparatus capable of avoiding a lock state of a wheel of an automobile wherein, a braking pressure is reduced when the wheel is about to enter a lock state at the time of the application of a braking force to the wheel, the braking pressure is returned when the revolution speed of the wheel is returned to the original speed by the reduction of pressure, and the above-mentioned operations are repeated.
2. Discussion of Background
There has been proposed antiskid apparatuses of this kind as shown in, for instance, Japanese Examined Patent Publication No. 28541/1986.
FIG. 5 is a diagram showing the operation for preventing a skid. In FIG. 5, a reference numeral 101 represents a wheel speed, numeral 102 indicates an actual vehicle body speed, numeral 103 designates a vehicle body speed which contains a slip factor of x %, a numeral 104 represents a deceleration of the wheel speed (hereinbelow, referred to as a deceleration) and numeral 105 indicates a braking pressure.
As is clear from FIG. 5, the operations for avoiding the antiskid in the conventional antiskid apparatus are carried out as follows. When the wheel deceleration 104 decreases to a predetermined value .alpha. 1 or lower, the braking pressure 105 is once maintained (during the time t1-the time t2). When an amount of slip of the wheel increases, a pressure reducing operation is started (at the time t2). The pressure reducing operation continues until the time point t3 when the deceleration 104 exceeds a predetermined value .alpha.1. Namely, the initiation of the pressure reduction is effected in accordance with the logical product of the deceleration and the slip factor, and the termination of the pressure reduction only relys on the deceleration.
As another conventional antiskid apparatus, there is a proposal as shown in Japanese Examined Patent Publication 222269/1988. In the antiskid apparatus, a wheel acceleration V.sub.w(n) is calculated with use of a digital filter in which calculations as in the following formula are conducted; ##EQU1## where V.sub.x(n) : instantaneous acceleration, and a.mu., b.gamma.: digital filter coefficients. In this formula, the digital filter coefficients a.mu., b.gamma. are respectively determined so as to depend on the wheel speed.
The conventional antiskid apparatus having the construction as described above has a noise removing function. Namely, it is required to remove noise components since a wheel speed signal contains various high frequency noise components. In the conventional antiskid apparatus, therefore, the noise components were removed by using an AND circuit in which the logical product of the wheel deceleration and the slip factor is used, or by using a digital filter to treat the wheel deceleration.
In the conventional antiskid apparatus, however, since the noise removing treatment was carried out without considering the friction between the tire of the wheel and a road surface conditions of which changes from time to time during the breaking operation to the vehicle, the following problem arised. Namely, when the brake control performance is set on a low friction side L.mu., an excessive reduction of brake pressure was found when the vehicle is in a high friction state H.mu., and when the brake control performance is sifted to the high friction side H.mu., the pressure reduction quantity was too small to thereby cause a large slip.